The present invention relates to a process for automatically controlling the combustible concentration during firing of ceramic products. In particular, the invention relates to a process for maintaining an acceptable percentage of the lower flammability limit (LFL) during organic additives burnout region in ceramic products.
During firing of ceramic products organic additives vaporize into the kiln atmosphere. These vapors include hydrogen and carbon monoxide, which are combustible and can become flammable leading to dangerous conditions during processing.
The National Fire Protection Agency (NFPA) requires manufacturers of ceramic products with organic additive systems to maintain kiln atmospheres at specific levels of volatile organic compounds to prevent uncontrolled combustion, deflagration or detonation. The minimum concentration of volatile combustibles in which a flame can be propagated is known as the Lower Flammability Limit or LFL (also referred to as xe2x80x9cLower Explosive Limitxe2x80x9d, LEL) and has units of percentage. In particular a LFL level of 70% means that the atmosphere contains a combustible volatile compound or mixture of combustible volatile compounds in a concentration equal to 70% of the composite Lower Flammability Limit of the mixture. At 100% LFL the atmosphere can sustain and propagate a flame.
Accordingly it is an object of this invention to provide a process of, efficiently and effectively controlling the flammable or combustible concentrations in a kiln atmosphere during firing of ceramic products containing organic additive systems.
The process of the present invention comprises establishing a maximum percentage of the lower-flammability limit (LFL) setpoint no greater than 50%; measuring continuously the percentage of LFL in the kiln atmosphere; and, maintaining the measured percentage of LFL to be less than or equal to the maximum percentage of LFL setpoint by an action selected from the group consisting of increasing gas volume delivered to the kiln, decreasing O2 concentration in the kiln, decreasing heating rate in the firing cycle, and combinations thereof.
Preferably the maximum percentage of LFL setpoint is no greater than 30%-40%.